1. Field of the Invention
The present invention relates to techniques for monitoring temperature and pressure in a remote and hostile environment. More particularly, the present invention is directed to reliably measuring downhole fluid pressure and temperature within a borehole of an oil, gas, or geothermal well.
2. Description of the Background
The accurate measurement of downhole fluid pressure and temperature in a borehole has long been recognized as being important in the production of oil, gas, and/or geothermal energy. Accurate pressure and temperature measurements typically indicate a number of problems in pumping wells which are commonly used in oil recovery operations. Both secondary hydrocarbon recovery operations and geothermal operations typically require pressure and temperature information to determine various factors considered useful in predicting the success of the operation, and in obtaining the maximum recovery of energy from the borehole.
In secondary hydrocarbon recovery operations, accurate borehole pressures specifically give an indication of well productivity potential, and allow the operator to predict the amount of fluid that should be required to fill the formation before oil or gas can be expected to be forced out from the formation into the borehole and then recovered to the surface. In geothermal wells, accurate pressure and temperature information is critical to efficient production due to the potential damage which occurs if reinjected fluids cool the formation. As a further example, the accurate measurement of pressure and temperature changes in well fluids from each of various boreholes extending into a formation may indicate the location of injection fluid fronts, as well as the efficiency with which the fluid front is sweeping the formation.
Techniques have been devised for providing a periodic measurement of downhole conditions by lowering sensors into the borehole at desired times, although such periodic measurement techniques are both inconvenient and expensive due to the time and expense normally required to insert instrumentation into the borehole. Any such periodic measurement technique is limited in that it provides only a representation of borehole conditions at specific times, and does not provide the desired information over a substantial length of time which is typically desired by the operator. An example of this type of system is disclosed in U.S. Pat. No. 3,712,192, which teaches charging an open-ended tube with a gas until it bubbles from the bottom of the tube in order to provide the desired periodic pressure measurement.
Permanent installation techniques have been devised for continually monitoring pressure in a borehole in a manner which overcomes the inherent problems associated with periodic measurement. One such prior art technique employs a downhole pressure transducer and a temperature sensor having electronic scanning ability for converting detected downhole pressures and temperatures into electronic data, which then are transmitted to the surface on a conductor line. The conductor line is normally attached to the outside of the tubing in the wellbore, and the transducer and temperature sensor are conveniently mounted on the lower end of the production tubing. This system has not, however, been widely accepted in the industry, in part because of the expense and high maintenance required for the electronics positioned in the hostile wellbore environment over an extended period of time. The high temperatures, pressures and/or corrosive fluids in the wellbore thus substantially increase the expense and decrease the reliability of the downhole electronics. Downhole pressure transducers and temperature sensors which output electronic data for transmission to the surface are generally considered delicate systems, and thus are not favored in the hostile environments which normally accompany a downhole wellbore.
U.S. Pat. No. 3,895,527 discloses a system for remotely measuring pressure in a borehole which utilizes a small diameter tube which has one end exposed to borehole pressure and has it other end connected to a pressure gauge or other detector at the surface. The concept of measuring downhole pressure according to a system which uses such a small diameter tube is also disclosed in U.S. Pat. No. 3,898,877, and an improved version of such a system is disclosed in U.S. Pat. No. 4,010,642. The teachings of this latter patent have rendered this technology particularly well suited for more reliably measuring pressure in a borehole, since the lower end of the tube extends into a chamber having at least a desired volume which satisfies the relationships expressed in the '642 patent.
Although the techniques disclosed in the '642 patent have been accepted in the energy recovery industry, the teachings of this patent do not enable the detection of both downhole temperature and pressure at the desired location within the borehole. In general, an operator may estimate downhole fluid temperature by either extrapolating from assumed temperature gradient data and temperature measurements taken at the surface, and/or by estimating an average temperature for the borehole from previously obtained drilling data. This estimated temperature may then be used to determine a test fluid correction factor, which may then applied to more accurately determined downhole pressure. Those skilled in the art have long recognized, however, that accurate temperature information is not being obtained, and that the correction of pressure readings based on such inaccurate temperature estimates accordingly results in errors in the pressure readings obtained by the technique of utilizing such a small diameter tube.
The disadvantages of the prior art are overcome by the present invention, and improved methods and apparatus are hereinafter disclosed for reliably detecting both pressure and temperature in a wellbore utilizing a single small diameter tubing extending from the surface of the well to the desired downhole test location.